Recent improvements in lighting technology have developed white solid-state lamp technology based on the use of a blue or ultraviolet AlGaN light-emitting diodes (LEDs) or laser diodes. These devices offer the exciting potential of highly efficient low voltage lighting sources that are rugged, highly reliable, and inexpensive. For highly industrial countries, the potential energy savings are very significant. In the U.S., about 20% of all electricity and about 7.2% of all energy is used for lighting. Energy savings also can result in environmental improvements by lowering the emissions from coal or oil fired power plants. Low voltage solid-state lighting also offers the opportunity to take advantage of local power sources, reducing the need for expensive power grids. Low voltage solid-state lighting offers a wide range of new lighting sources and products, including distributed panel lighting, conformable lighting systems, and intelligent lighting schemes.
A white solid-state lamp can be obtained by coating a conventional AlGaN diode with a phosphor. The phosphor is chosen so that it absorbs strongly in the regions of the diode emission (blue or UV) and efficiently transfers this energy to an activator that emits light in the visible spectrum, such as green or red, or a combination of green and red. One phosphor successfully used is yttrium aluminum garnet:cesium doped (YAG:Ce) phosphor. YAG:Ce phosphor has the advantage that the cesium activator strongly absorbs in the blue region and internally down converts this radiation into a broad yellowish spectrum, which combines with the blue pump light from the LED to produce a white spectrum. Other potential phosphor systems can use two activators or be excited in the blue or UV region. Additionally, several phosphors can be mixed to give a white light.
FIG. 1 shows a schematic diagram of a typical flat LED mounted in a reflective cup. The LED chip 80, having a top 81 and sides 82, comprises a p-side 84, an active region 85, and an n-side 86. A first leadframe 88 and second leadframe 90 can provide electrical connections between the LED chip 80 and a circuit board (not shown). The LED chip 80 is disposed in a reflective cup 92 in the first leadframe 88 to reflect light generated by the LED chip 80. The first leadframe 88 can be electrically connected to the n-side 86 directly by contact or wirebonded. The second leadframe 90 can be electrically connected to the p-side 84 by gold wire 94 at the top or side of the LED chip 80.
To achieve light emission, the LED chip 80 is typically forward biased by 2 to 4 Volts, equivalent to the band gap energy of the semiconductor, i.e., the p-side 84 is held at a positive 2 to 4 Volts over the n-side 86. In general, light emission occurs from the p-side 84 of the LED chip 80 and is emitted most intensely from the sides 82 of the LED chip 80, and less intensely from the top 81 of the p-side 84.
Rather than the flat LED chip illustrated in FIG. 1, the LED chip can have an inverted trapezoidal geometry, with the large face of the trapezoid on the top, so that the light generated within the p-side reflects internally and passes upwards from the LED chip. The inverted trapezoidal geometry has the disadvantage of requiring extra diode material to achieve the proper reflection angle. The trapezoidal, or any other externally shaped LED chip, can be used with or without a reflective cup.
The commercial technique typically employed in phosphor deposition on LEDs involves the use of phosphor powders blended in a liquid polymer system, such as polypropylene, polycarbonate or, more commonly, epoxy resin, or silicone. Generally, a small amount of the phosphor-impregnated epoxy is simply painted or dispensed on the LED die, then dried or cured. A clear epoxy lens is then constructed around the die, although the phosphor-impregnated epoxy can be used to construct the whole LED lens. Other techniques have also included dusting phosphor powders or spray painting phosphor powders liquid mixtures directly on the LED die.
Current phosphor deposition methods are inefficient in production and less than optimum in result. Rather than selectively coating only the light emitting regions of the diode, the phosphor is deposited over the whole diode package. Much of the phosphor is wasted, washing off during application and requiring retrieval later. The phosphor does not make good contact with the diode surface in the most desired locations for efficient energy transfer from the diode to the phosphor. In addition, the current phosphor deposition methods are difficult to translate into mass production for coating many single diodes and for coating large arrays of diodes mounted on circuit or ceramic boards.
The resulting white solid-state lamps may lack color repeatability and uniformity, so as to be unsuitable for color-critical applications. The lamps may be inefficient and convert less of the chip radiation into visible light than possible due to phosphor placement away from the light emitting regions of the diode, and absorption and reflection in binder materials.
It would be desirable to have a method for selectively depositing materials on a semiconductor device that would overcome the above disadvantages.